Many types of implantable medical devices (IMDs) have been implanted that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., ventricular tachycardia or ventricular fibrillation, is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are typically delivered when fibrillation criteria are met and an R-wave cannot be discerned from the electrogram (EGM).
The current state of the art of ICDs or implantable pacemaker/cardioverter/defibrillators (PCDs) includes a full featured set of extensive programmable parameters which includes multiple arrhythmia detection criteria, multiple therapy prescriptions (for example, stimulation for pacing in the atrial, ventricular and/or both chambers, bi-atrial and/or bi-ventricular pacing, arrhythmia overdrive or entrainment stimulation, and high level stimulation for cardioversion and/or defibrillation), extensive diagnostic capabilities and high speed telemetry systems.
Current technology for the implantation of an IMD uses a transvenous approach for cardiac electrodes and lead wires. The defibrillator canister/housing is generally implanted as an active can for defibrillation and electrodes positioned in the heart are used for pacing, sensing and detection of arrhythmias.
Attempts are being made to identify patients who are asymptomatic by conventional measures but are nevertheless at risk of a future sudden death episode. Current studies of patient populations, e.g., the MADIT II and SCDHeFT studies, are establishing that there are large numbers of patients in any given population that are susceptible to sudden cardiac death, that they can be identified with some degree of certainty and that they are candidates for a prophylactic implantation of a defibrillator (often called primary prevention).
One option proposed for this patient population is to implant a prophylactic subcutaneous implantable device (SubQ device). As SubQ device technology evolves, it may develop a clear and distinct advantage over non-SubQ devices. For example, the SubQ device does not require leads to be placed in the bloodstream. Accordingly, complications arising from leads placed in the cardiovasculature environment are eliminated. Further, endocardial lead placement is not possible with patients who have a mechanical heart valve implant and is not generally recommended for pediatric cardiac patients. For these and other reasons, a SubQ device may be preferred over an ICD.
There are technical challenges associated with the operation of a SubQ device. For example, SubQ device sensing is challenged by the presence of muscle artifact, respiration and other physiological signal sources. This is particularly because the SubQ device is limited to far-field sensing since there are no intracardial or epicardial electrodes in a subcutaneous system. Further, sensing of atrial activation from subcutaneous electrodes is limited since the atria represent a small muscle mass and the atrial signals are not sufficiently detectable transthoracically.
Yet another challenge could occur in situations where it is desirable to combine a SubQ device with an existing pacemaker (IPG) in a patient. While this may be desirable in a case where an IPG patient may need a defibrillator, a combination implant of a SubQ device and an IPG may result in inappropriate therapy by the SubQ device, which may pace or shock based on spikes from the IPG. Specifically, each time the IPG emits a pacing stimulus, the SubQ device may interpret it as a genuine cardiac beat. The result can be over-counting beats from the atrium, ventricles or both; or, because of the larger pacing spikes, sensing of arrhythmic signals (which are typically much smaller in amplitude) may be compromised.
Therefore, for these and other reasons, a need exists for an improved method and apparatus to reliably sense and detect arrhythmias in a subcutaneous device, while rejecting noise and other physiologic signals.